The present invention relates to a hydraulic control system, and more particularly to a hydraulic control system of the type having a source of pressure fluid and a control valve which supplies pressure fluid from the source to a double-acting user and returns the pressure fluid from the user into a reservoir.
Hydraulic control systems of this general type are already known. A prior-art system uses a four-port, three-position slide valve which is connected with a double-acting user by means of two hydraulic fluid conduits. Located ahead of the slide valve is a pressure reducing valve to assure that the user can be controlled at a pressure that is lower than the pressure fluid supplied by the pump of the system.
This prior-art hydraulic control system is fully operative but is relatively complicated and expensive. In certain applications, in which a load usually acts upon the user in a predetermined direction, such systems are more complicated and expensive than the requirements of the application warrant, which is particularly true when an undesired descending of the load is to be prevented, under which circumstances a control valve must be used of the type having valve seats and a cooperating valve member.
Another hydraulic control system known from the prior art is used for lifting and lowering a hydraulically powered arm of an agricultural machine, such as a tractor or the like. In that system there is a single-acting cylinder provided the operation of which is controlled by valves of the type having seats and cooperating valve members. This is a simple system in its construction, and serves to reliably seal the fluid pressure space in the cylinder. However, in this particular system, it has been found that if there is no load actng upon the arm which is to be controlled, the inherent friction of the arm and the associated components may be sufficiently high so that in the absence of a load, the arm will descend either only very slowly or not at all, thus making the arrangement quite difficult to use when this occurs.